In order to study the freezing/melting behavior of pore water, we performed x-ray diffraction measurements of water confined inside the cylindrical pores of two kinds of siliceous MCM-41 with different pore size and one kind of aluminosilicate MCM-41 as a function of temperature. The results show that its freezing/melting behavior is not affected by the incorporation of Al into the pore wall and the hysteresis effect between freezing and melting is very small or negligible. On cooling the water in the middle of the pores with a pore diameter of 4.2 nm, that is, the free water freezes abruptly around 232 K to give rise to cubic ice while the water confined in the pores with a pore diameter of 2.4 nm freezes very gradually at lower temperatures. The diffraction profile after the freezing of the free water suggests that the interfacial water confined between the surface of the pore wall and the frozen phase of the free water consists of randomly displaced water molecules. (C) 1997 American Institute of Physics. [S0021-9606(97)51841-1].